Biswabrata Goswami
PURULIA, 15 JAN: At daybreak, when the Ajodhya Hill Range usually wakes to birdsong and mist, morning walkers in Begunkodar, Jhalda and Bagmundi were greeted this week by an unfamiliar sight — thin sheets of white crystals clinging to grass, leaves and the bare earth. For many locals, it looked like ice. For some, almost like a freak snowfall. Confusion travelled faster than the winter breeze across the hill villages of western Purulia.
But science, not snowfall, lay behind the spectacle.
Explaining the phenomenon, Dr Biswajit Bera, Professor at Sidho-Kanho-Birsha University, Purulia, said the white layer was hoar frost, also known as ground frost — a natural and recurring winter occurrence in hilly terrains like the Ajodhya Hills. Similar incidents, he noted, have been recorded earlier in the Begunkodar region.
“People often associate ice on the ground with extreme cold or snowfall. In reality, this is a surface-level atmospheric process,” Dr Bera explained.
According to him, ground frost forms when the surface temperature of the ground or objects such as grass, soil, rooftops or vehicle windows falls below the freezing point, even if the air temperature measured at the standard height of two metres remains slightly above zero. Under such conditions, water vapour in the air turns directly into ice crystals through sublimation, bypassing the liquid stage altogether.
Several factors came together to create the perfect conditions for frost formation in Purulia’s hill tracts.
First, radiational cooling played a decisive role. Clear winter nights allow the Earth’s surface to lose heat rapidly through long-wave radiation. “Cloud cover acts like a blanket. When skies are clear, the ground cools very fast,” Dr Bera said.
Second, the region experienced calm or near-calm winds, preventing warmer air from mixing with the colder, denser air that settles close to the ground. This creates a thin, super-cooled layer right at the surface, ideal for frost development.
Third, adequate moisture in the air supplied the raw material for ice crystal formation. Even moderate humidity is enough when temperatures at the surface dip below freezing.
Dr Bera also pointed to the possibility of a temperature inversion, or negative lapse rate, during early morning hours — a situation where air near the ground becomes colder than the air above it, further intensifying frost formation.
The Ajodhya Hill Range, with its undulating terrain, forest cover and elevation, is particularly prone to such microclimatic behaviour during winter nights.
While the sight caused momentary alarm among residents, experts stress that ground frost is harmless and short-lived, disappearing soon after sunrise as temperatures rise.
For scientists, it is a reminder of the region’s sensitive microclimate. For early risers in Purulia’s hills, it was a fleeting brush with winter’s quiet magic — a white illusion born not of snow, but of science.
